What's the best way to bring two channels together?

[dehughes]

Hello all,

I'm looking at bringing two distinct channels together in one amp, and I have two questions:

1) Is it "safe" to just have one input driving two separate channels at the same time, thus allowing you to blend the two channels from the one input? Each channel would have two triodes, thus keeping the signal in phase. Any recommendations as to how one should go about that, other than the obvious...?

2) What's the best way to bring these two channels into the PI? Should I use a pair of mixer resistors (a la the AB763 Deluxe...) and bring them into one side of the PI, or is it "better" to bring one channel to each side of the PI?

Thanks so much!

[mlp-mx6]

Another option, though expensive...

If you have a spare 12A*7, you can feed each signal to a separate grid, while sharing the plate resistor (and optionally the cathode also). Kevin O'Connor calls this a "tweed mixer" - I can say that it works well to blend signals, if you have a tube to spare.

[oldhousescott]

If both channels are truly in phase, then the best course would be to use mix resistors, or the tweed mixer MLP-MX6 mentioned. If you bring in-phase signals into separate sides of the PI, they will tend to cancel each other.

[David Root]

+1 on the tweed mixer. Guaranteed no crosstalk between channels.

If it goes directly into the PI, fine. If it goes to a tonestack first I'd put a cathode follower in between to minimise gain loss in the tonestack.

[dehughes]

Hmm...thanks. I don't have a spare 12AX7 slot...so perhaps I'll have to do the mixer resistor then?

[klingo]

if you have a cathode follower you may DC mix your channels in

http://www.lynx.net/~jc/TweedReverbPreamp.gif

[klingo]

I have also used both input of the PI and it work nicely:

if you are using a big tail resistor with a "log tailed pair" PI/no neg feedback/same plate resistors value (vox or 18w).

you may need input resistor (39k in my case) on each input to help with phase/overdrive issue.

both inputs seeing out of phase signal.

[dehughes]

The PI is essentially a 6G3, save for no feedback resistor from the OT secondary. I'm thinking that I'll just use the "top half" and just use a couple of 220k resistors and call it good. The signals are going to be in phase as I'm using two triodes for each side of the preamp, so I wager it'll be fine...(fingers crossed...).

I do have one question, though: In the 6G3 you just have the PI cap connected right to the plate of the preceding triode...no cap/resistor, just the .01uf cap connected between the 100k plate resistor and a 15k resistor. However, in the non-reverb AB763 Deluxe, you have the 100k plate resistors on each channel, then a cap for each channel (.1 and .047), THEN the 220k mixer resistors. Would I need to have a cap off the plate of each triode a la the AB763, then, prior to the mixer resistors, or could I just install the mixer resistors after the triodes without any prior cap? Or, are the caps in the AB763 schematic just there to trim out some low end?

[mlp-mx6]

If I'm understanding your question correctly - yes, you must have those caps.

[oldhousescott]

Because of the way the intensity pot ties into the vibrato channel signal on the AB763, the mix resistors have to come after coupling caps. In your case, however, you could have your mix resistors coming directly off your plates feeding the PI input cap.

[dehughes]

Because of the way the intensity pot ties into the vibrato channel signal on the AB763, the mix resistors have to come after coupling caps. In your case, however, you could have your mix resistors coming directly off your plates feeding the PI input cap.

Yeah, my idea is to basically mix the normal channel of a Brown Deluxe preamp with an octal tube (similar to what the Komet Constellation does...) at the PI input. NO vibrato, reverb, etc... Do I need coupling caps of the plates and before the mixer resistors, then, or can I just run the plates into mixer resistors and then into the PI?

http://www.ampwares.com/ffg/schem/deluxe_6g3_schem.gif

http://www.ampwares.com/ffg/schem/delux ... _schem.gif

The 6G3 just goes from the plate into the PI...no cap. The AB763 runs each channel through a cap off the plates, THEN into the PI. Hence my question.

[jjman]

In both schematics the 2- 220k mixing resistors do not see any DC. I would be afraid to connect some of the DC of the one plate to some of the DC of the other plate which would happen if you did not use caps before the mixing resistors. In addition, you could fine tune each preamp by using different value caps for each, as seen in the AB763’s use of the 0.1 and the 0.047.

I am no expert by far but no caps before the mixers seems like it could cause some sort of undesirable interaction between the preamps.

[dehughes]

Interesting...some say that caps are needed, while others say they are not needed. I wager there is a definitive answer...I just don't know what it is.

I do know that the 6G3 does not have a cap on the last triode before the PI...but instead connects the PI cap off the top of the plate resistor. However, this channel is the only one going into the PI, as the other channel was mixed before that last triode. So, in my case, what should I do when mixing two channels into the PI input cap? There'll be DC going from channel to channel, plate to plate, if I just use mixer resistors right off the plates, yes? Is that a problem?

Thanks so much for your help, guys!

[rhinson]

Interesting...some say that caps are needed, while others say they are not needed. I wager there is a definitive answer...I just don't know what it is.

I do know that the 6G3 does not have a cap on the last triode before the PI...but instead connects the PI cap off the top of the plate resistor. However, this channel is the only one going into the PI, as the other channel was mixed before that last triode. So, in my case, what should I do when mixing two channels into the PI input cap? There'll be DC going from channel to channel, plate to plate, if I just use mixer resistors right off the plates, yes? Is that a problem?

Thanks so much for your help, guys!

use the caps in your case---it'll give you the opportunity to further voice the channels differently. take a look at the 5e8a tweed twin schematic as well. you don't necesarily need them since the p.i. has a dc blocking cap, but there may be interaction of the channels that you don't like. if you're making boards, i'd make a spot there for both and just use a jumper if you decide you like no caps better and just resistor couple them.

rh

[dehughes]

Okay. Fair enough. What are the supposed differences in terms of interaction between caps and no-caps?